A ‘loophole’ is something that is expressly legal until it’s used in a way we dislike – or in this case, something that airline competitors and unions don’t like. The common connotation of loophole is some sort of mistake or drafting accident that’s being exploited.
However what JSX and others do by operating charter flights under FAA part 135, and selling individual seats on those flights under Part 380, is not actually a loophole. The linkage between parts 135 and 380 in regulations is actually intentional.